Dithiocarbamate pesticides are broadly used as antimicrobial agents or nematicides in the fields of pesticides and are also used in fields involving medicinal crude drugs that are raw materials for kampo preparations.
A carbon disulfide method (that is, a typical method for analyzing a dithiocarbamate pesticide) is a technique that involves heating a dithiocarbamate pesticide remaining in a sample in an acid catalyst, digesting it into carbon disulfide, measuring the level of carbon disulfide, and then quantitatively determining the level of the residual pesticide in terms of carbon disulfide. This method requires the use of special glassware for digestion and various powerful drugs (hydrochloric acid, concentrated sulfuric acid, and alkali) and is complicated as an analytical method. Another example is a methylation method that involves derivatizing a dithiocarbamate pesticide and then quantitatively determining the derivative. Such methylation method is better than the carbon disulfide method since it allows species of residual pesticides (ethylenebisdithiocarbamates, dimethyldithiocarbamates, and propylenebisdithiocarbamates) to be determined, but is almost the same as the carbon disulfide method in terms of maneuverability and ability for specimen treatment. Also, because of the physical properties (degradability and poor solubility) of dithiocarbamate pesticides, it is difficult to apply thereto a simultaneous analytical method that has generally been employed in recent years and enables simultaneous confirmation of many residual pesticides.
Although the amounts of dithiocarbamate pesticides used are high, there is no choice but to depend on a separate analytical method with poor efficiency of residue analysis because of the above reasons. Hence, residual pesticide control has been considered to be difficult in the case of dithiocarbamate pesticides.
J. Agric. Food Chem., 2002, 50, 2220-2226 discloses a method that involves heating by microwave irradiation a dithiocarbamate pesticide remaining in dry tobacco or peach, so as to digest it into carbon disulfide and then quantitatively determining the carbon disulfide. This analytical method is an extremely simple technique comprising: adding a specimen, 5 mol/L hydrochloric acid, tin (II) chloride as a catalyst, and isooctane as an organic solvent into a resin cylinder, sealing the cylinder, heating the cylinder using microwaves, so as to digest the remaining dithiocarbamate pesticide into carbon disulfide, carrying out fractional extraction using an organic solvent within a reaction cylinder, collecting the organic solvent phase, and then analyzing it using a gas chromatograph (GC-FPD) provided with a flame gas photometric detector.
However, when the present inventors applied the analytical method as described in J. Agric. Food Chem., 2002, 50, 2220-2226 to crude drugs, kampo preparations, and the like, the addition and recovery test for pesticides yielded insufficient results suggesting a need of further improvement.
J. Agric. Food Chem., 1992, 40, 76-80 describes that L-cysteine prevents digestion of mancozeb (manzeb), which is an ethylenebisdithiocarbamate pesticide into ethylenethiourea. However, the document mentions nothing about digestion of an ethylenebisdithiocarbamate pesticide, a propylenebisdithiocarbamate pesticide, or a dimethyldithiocarbamate pesticide into carbon disulfide.
Lecture Summaries of the 29th Pesticide Residue Analysis Committee, 2006, pp. 168-171 describes that: when fruit or vegetable samples are ground, dithiocarbamate pesticides are immediately digested through their contact with fluids; and when L-cysteine is added to such samples, digestion of dithiocarbamate pesticides is suppressed. However, the document does not clarify what kind of degradation reaction is suppressed and suggests nothing about how L-cysteine acts when it is caused to coexist in the microwave-assisted system for digestion reaction of dithiocarbamate pesticides into carbon disulfide. Also, in Lecture Summaries of the 29th Pesticide Residue Analysis Committee, 2006, pp. 168-171, the effects of L-cysteine have been confirmed for Citrus natsudaidai, grape, apple, and lettuce samples, but not confirmed for grape tomato samples.